Checkboxes in a Tree Control

Abstract

"Checkboxes in a tree control" shows how to add normal and three-state checkboxes to a tree control, and how to get notified when the checkbox changes. The sample project uses MFC, but the technique can easily be adapted to a Win32 or an ATL/WTL application.

Overview

The TreeView common control supports checkboxes since version 4.70 (distributed originally with Internet Explorer 3.0), and enabling this is supposedly easy- choose one of three:

• use the Checkboxes option in dialog resource editor
• specify the TVS_CHECKBOXES window style bit when creating the tree
• Use
    treeCtrl.SetWindowStyle(0, TVS_CHECKBOXES)
  in MFC applications, or
    SetWindowLong(hwndTree, GWL_STYLE, GetWindowLong(hwndTree, GWL_STYLE) | TVS_CHECKBOXES);
  InvalidateRect(hwndTree, NULL, true)
  for Win32 applications. This must be done before the tree control is filled with items.

However, there are a few problems asociated with this:

• How do I detect when the checkbox is changed
• How (and when!) do I retrieve the current check state
• How to use Three-State Checkboxes.

Tree Control Images

The tree control uses the state image list to implement the checkboxes. When the TVS_CHECKBOXES style is set, the tree control creates a custom state image list with the two state image buttons - unchecked & checked. When clicking on the state image, the tree control cycles through the state images.

Retrieving and Setting State Images

void TreeCtrl_SetStateImage(CTreeCtrl & tree, HTREEITEM item, int stateImage)
            {
            tree.SetItemState(item, INDEXTOSTATEIMAGEMASK(stateImage), TVIS_STATEIMAGEMASK);
            }

int TreeCtrl_GetStateImage(CTreeCtrl & tree, HTREEITEM item)
            {
            return tree.GetItemState(item, TVIS_STATEIMAGEMASK) >> 12;
            }

Note that 0 indicates no state image; so you can use bool isChecked = (TreeCtrl_GetStateImage(...) - 1) != 0.

Three-State Check boxes

For three-state checkboxes, we just need to replace the default created state image list with our custom image list, windows will figure out the number of check states from the length of the image list. Remember that the first image (index 0) is unused. The sample uses the following Bitmap resource (IDB_STATEIMG in the sample):

• Add a member variable, CImageList m_ilState to the dialog
  (the image list must not be destroyed as long as the tree control is displayed)
• In OnInitDialog, add the following:
  m_ilState.Create(IDB_STATEIMG /* resID of bitmap */,
                   16 /* width of single image */,    0 /* 0: the image list won't grow */,
                   RGB(255,255,255) /* transparent color */);
  m_treeCtrl.SetImageList(&m_ilState, TVSIL_STATE); // set state image list

The size of the bitmap affects the number of checkbox 'states'. To retrieve the current state, use TreeCtrl_GetStateImage(...) - 1. As with a checkbox, this value is 0 for unchecked, 1 for checked, and 2 for indeterminate.

This way, you could create a 'checkbox' with up to 15 states - which, on the other hand, ounds notlike fun for your users. More promising is the idea that you are not limited to checkboxes!

Being Notified when Checkbox changes

We want to know when the checkmark is clicked,so we can do something. The TreeView control does not send a separate notification, when item state changes, so we must manage both NM_CLICK wnd TVN_KEYDOWN message separately.

It's even trickier: When you get these notifications, the item state bits still indicates the old state. This wouldn't be as bad itself, but Microsoft might be tempted to change this behavior in the future. You can get the correct image (and thus, the current check state) when the current "Click" or "KeyDown"  message is completely handled. To do this, we can post a message to outselves, using PostMessage(some_unique_message_id, ..), and evaluating the new style there. With the message, I send the tree control ID in WPARAM (in case there are multiple tree controls in the dialog), and the item which has changed in LPARAM.

// .h:
            
            #define UWM_TV_CHECKBOX    WM_APP   // the custom message we post to ourself
            class CDlgOrWhatever
            {
            // ...
            CTreeCtrl    m_tree;
            afx_msg LRESULT OnTvCheckbox(WPARAM wp, LPARAM lp); // our message handler for the posted message
            };

// .cpp
            #include "windowsx.h"  // required for GET_X_LPARAM, GET_Y_LPARAM)

// ----- Message map ------
            BEGIN_MESSAGE_MAP(...)
            ON_MESSAGE(UWM_TV_CHECKBOX, OnTvCheckbox)
              // ... and, of course, the message handlers for NM_CLICK and TVN_KEYDOWN notifications
            END_MESSAGE_MAP()

// ----- NM_CLICK Message Handler: ------
            void CCheckTreeDlg::OnClickTree1(NMHDR* pNMHDR, LRESULT* pResult)
            {
            DWORD dw = GetMessagePos();                   // retrieve mouse cursor position when msg was sent
            CPoint p(GET_X_LPARAM(dw), GET_Y_LPARAM(dw)); // ..and put into point structure
            m_tree.ScreenToClient(&p);                    // make coords local to tree client area
            UINT htFlags = 0;
            HTREEITEM it = m_tree.HitTest(p, &htFlags);   // See where the click was on
            if (it != NULL && ( htFlags & TVHT_ONITEMSTATEICON)) {
                // the check box was hit - we just post a message for later processing
                PostMessage(UWM_TV_CHECKBOX, pNMHDR->idFrom, (LPARAM) it);
            }
            *pResult = 0;
            }

// ----- Handle checkbox changed here: ------
            LRESULT CCheckTreeDlg::OnTvCheckbox(WPARAM wp, LPARAM lp)
            {
            CTreeCtrl & tree = GetDlgItem(wp);  // if we have multiple trees
            HTREEITEM hitem = (HTREEITEM) lp;
            int checked = (tree.GetItemState(hitem, TVIS_STATEIMAGEMASK) >> 12) - 1;
              // "checked" now contains the check state.
            // the sample does the following:
              CString s = tree.GetItemText(hitem);
            if (checked==0) s += " unchecked";
            else if (checked==1) s+= " checked";
            else if (checked==2) s+= " dunno";
            else s+= " ???";
            SetDlgItemText(IDC_INFO, s);
            return 0;
            }
            

Notes:

19.2.2002: brushed up formatting. Fixed an alignment error in the resource bitmap 8the image on this page is still wrong, but that doesn#t mater much I guess). Added the TVN_KEYDOWN handler to the sample to catch the state change correctly when a key is hit.

30.11.2002: fiexd a bug in the OnClick Handler (thanks Michael!)

posted on 2007-07-18 16:49 旅途 閱讀(2693) 評論(0)  編輯 收藏 引用 所屬分類: 深入windows